1. Field of the Invention
This invention relates to the field of data and voice communications and more particularly to a method and system for maintaining voice communication in an ISDN type modem system during power fail conditions.
2. Prior Art
ISDN modems are a known means of combining voice and data communication over telephone lines. ISDN (for "Integrated Services Digital Network") modems are primarily digital devices which convert analog voice signals to digital data packets which are combined with other digital information data packets for communication over conventional telephone lines. Since ISDN modems transmit and receive voice signals via digital means, in many systems,.voice communication becomes impossible when power is lost at a subscriber location.
Prior systems have been developed to provide fail-safe voice communication when power is lost at a subscriber location. For example, in a digital communication system described in U.S. Pat. No. 4,853,949--Schorr et al., telephone system central office circuitry monitors loop current, loop DC continuity, incoming ringing from the telephone system central office switch and synchronization between the various digital devices in the system. Under normal conditions, the respective digital devices are in an idle state. In this state, although handshaking signals are exchanged, no voice or data link is established. When the subscriber location telephone device(s) go off-hook, the respective digital transceivers in the system become active and a full transmission path is established. Alternatively, if an incoming ringing signal is detected at the telephone system central office from the central office switch, the respective digital transceivers also become active. Under normal conditions, no DC path between the respective digital transceivers is established.
When a power failure occurs at the subscriber location during a telephone call, a relay at the subscriber location bypasses the digital transceiver and the telephone device is directly coupled to the telephone line. As a result the telephone system central office digital transceiver loses synchronization. A DC continuity path is detected, and a relay connects the incoming loop directly to the telephone system central office switch.
If a power failure occurs during an idle state of the telephone device, DC continuity is not detected. Thus, the telephone system central office does not go into a bypass mode. If the telephone device subsequently goes off-hook during a power failure, the telephone system central office equipment senses the DC continuity and bypasses the telephone system central office digital transceiver, therefore connecting the subscriber telephone device to the central office switch.
During a power failure, and with the telephone device in an idle state, incoming ringing at the central office activates a ringing detector which will then instruct the telephone system central office digital transceiver to send a "wake-up" signal to the subscriber location transceiver. In this state, the subscriber location digital transceiver cannot respond, eventually causing the telephone system central office digital transceiver to "time-out", thus bypassing the telephone system central office digital transceiver and passing the ringing signal directly to the subscriber location. The subscriber location telephone then rings. If the subscriber location telephone device goes on-hook during a power fail mode, DC continuity will cease so the telephone system central office transceiver will no longer be bypassed.
During the idle state of the telephone, restoration of power at the subscriber location immediately activates a local relay, thus eliminating the bypass of the subscriber location transceiver. The telephone system central office digital transceiver remains connected in the idle state.
If power is restored during a voice call, the bypass relay at the subscriber location is immediately activated, thus connecting the local transceiver which interrupts the loop current which is then sensed at the telephone system central office by the loop current detector. A bypass relay at the telephone system central office then changes state so that the telephone system central office digital transceiver is no longer bypassed. The telephone system central office digital transceiver then sends a "wake-up" signal to the digital transceiver at the subscriber location in an attempt to re-synchronize the link.
While achieving its stated purpose, the above described system has several problems. A standard requirement for local loop systems is to provide a DC sealing current to the loop. In the absence of sealing current, a poorly-conducting oxide film will develop over a period of time at the various non-soldered wire splices used in the system. A sealing current in the range of 10 mA is used to prevent oxidation and maintain low resistance across the non-soldered connections. The fact that the system of Schorr et al. relies upon a lack of DC continuity during normal operation, prevents the introduction of beneficial sealing current. This violates ISDN standards as well as normal practice.
Further, in the system of Schorr et al., if power is restored during a voice call, the subscriber location bypass relay will immediately be activated and cause a current interruption by breaking the direct path over the local loop. Any momentary open-loop condition will be very disruptive and most likely results in a lost call.
In another system, described in U.S. Pat. No. 4,575,584--Smith et al., a combined digital and analog telephone is normally connected to a digital telephone exchange (PBX) and operates in a digital mode. In the event of a power failure or a telephone exchange malfunction, the digital telephone is converted to an analog mode, is disconnected from the digital telephone exchange (PBX), and is connected to a local analog telephone line. When power is restored, the telephone reverts to a digital mode and is recoupled to the digital telephone exchange. While this system also provides a degree of fail safe operation, the system is complicated, requiring both digital and analog telephone connections for operation.
In view of the foregoing, no system is known for detecting power failure at a remote subscriber location having a digital multiplexer/modem and an associated analog telephone device wherein upon detection of a remote power failure, the respective digital circuitry of the system is bypassed and the remote analog telephone is coupled directly to the telephone system central office over an analog local loop. No prior system monitors power status of a remote subscriber location at the telephone system central office for the purpose of restoring normal operation when power has returned.